Polishing monitoring method, polishing end point detection method, and polishing apparatus

ABSTRACT

A method of monitoring a thickness of a conductive film on a substrate during polishing of the substrate with use of an eddy current sensor is provided. This method includes: polishing the conductive film by pressing the substrate against a polishing surface on the rotating polishing table; obtaining output signal of the eddy current sensor during polishing; calculating an amount of output adjustment of the eddy current sensor using the output signal obtained when the substrate is not present above the eddy current sensor; with use of the amount of output adjustment, correcting the output signal obtained when the substrate is present above the eddy current sensor; and monitoring the thickness of the conductive film based on the corrected output signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This document claims priorities to Japanese Application Number 2011-173792, filed Aug. 9, 2011 and Japanese Application Number 2011-253801, filed Nov. 21, 2011, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a polishing monitoring method and a polishing apparatus for monitoring a change in thickness of a conductive film formed on a surface of a substrate, such as a semiconductor wafer, during polishing.

The present invention also relates to a polishing end point detection method used in a polishing apparatus for polishing an object (a substrate), such as a semiconductor wafer, and more particularly to a polishing end point detection method using an eddy current sensor. The present invention further relates to a polishing apparatus capable of performing such a polishing end point detection method.

2. Description of the Related Art

With a recent trend toward higher integration and higher density in semiconductor devices, circuit interconnects become finer and finer and the number of levels in multilayer interconnect is increasing. In the process of achieving the multilayer interconnect structure with finer interconnects, film coverage (or step coverage) of step geometry is lowered through thin film formation as the number of interconnect levels increases, because surface steps grow while following surface irregularities on a lower layer. Therefore, in order to fabricate the multilayer interconnect structure, it is necessary to improve the step coverage and planarize the surface in an appropriate process. It is also necessary to planarize surfaces of semiconductor device so that irregularity steps formed thereon fall within a depth of focus in optical lithography. This is because finer optical lithography entails shallower depth of focus.

Accordingly, the planarization of the semiconductor device surfaces is becoming more important in fabrication process of the semiconductor devices. Chemical mechanical polishing (CMP) is the most important planarization technique. This chemical mechanical polishing is a process of polishing a substrate, such as a semiconductor wafer, with use of a polishing apparatus by placing the substrate in sliding contact with a polishing pad while supplying a polishing liquid containing abrasive grains, such as ceria (CeO₂), onto the polishing pad.

The polishing apparatus that performs the above-described CMP process has a polishing table having the polishing pad, and a substrate holder for holding the semiconductor wafer (substrate). The substrate holder is often called a top ring or polishing head. When polishing the substrate, the top ring presses a surface of the substrate against the polishing pad while a polishing liquid supply unit supplies the polishing liquid onto the polishing pad. The top ring and the polishing table are rotated independently to provide relative movement between the substrate and the polishing pad to thereby polish a film that forms the surface of the substrate.

This polishing apparatus is widely used in polishing of a conductive film, such as a barrier film or a metal film, formed on the surface of the semiconductor wafer (substrate). A polishing end point and a change in polishing conditions during polishing are determined based on a thickness of the conductive film. Therefore, the polishing apparatus typically has a film thickness detector for detecting the thickness of the conductive film during polishing. An eddy current sensor is typically used as the film thickness detector.

The eddy current sensor is provided in the polishing table. During polishing of the substrate, the eddy current sensor induces eddy current in the conductive film on the substrate while sweeping under the substrate as the polishing table rotates, and detects the thickness of the conductive film from a change in impedance due to a magnetic field of the induced eddy current.

FIG. 39 is a diagram showing a relationship between signal value of the eddy current sensor and polishing time (t) from when polishing of the semiconductor wafer (substrate) is started until the conductive film on the semiconductor wafer is cleared (removed). Immediately after polishing of the semiconductor wafer is started, the conductive film is still thick. As a result, the eddy current sensor outputs high signal as shown in FIG. 39. As the polishing process proceeds, the signal value of the eddy current sensor is lowered with the decrease in the thickness of the conductive film. When the conductive film is cleared (removed), the signal value of the eddy current sensor becomes constant. Therefore, it is possible to judge the polishing end point by detecting such a point of time (i.e., a singular point) when the signal value becomes constant.

However, a change in operating environment, such as ambient temperature of the eddy current sensor or permeation of the liquid into the polishing pad, or a change in the eddy current sensor itself with time may cause drift (i.e., translation) of the output signal value of the eddy current sensor. Such drift results in upward translation of a graph itself as indicated by a movement from a solid line to a dotted line depicted in FIG. 39. Even in such a case, it is possible to detect the polishing end point because the singular point is translated in the same manner. However, in a case of stopping the polishing process when the conductive film reaches a predetermined thickness so as to leave a part thereof, or in a case of switching to different conditions, e.g., lower pressure or lower rotational speed, it is necessary to detect a specific point where the signal value reaches a predetermined value (Z2). In such a case of detecting the specific point in accordance with the signal value, the drift can cause an error of the polishing time to be detected because of a change in the relationship between the output signal value of the eddy current sensor and the film thickness.

In fabrication process of semiconductor devices, several kinds of materials are deposited repeatedly in the form of film on a silicon wafer to form a multilayer interconnect structure. In order to form such a multilayer interconnect structure, CMP (chemical mechanical polishing) is used. For example, a metal film is formed on a surface of a substrate having interconnect trenches formed thereon, and then CMP is performed so as to remove unnecessary film, so that the metal film remains only in the trenches to form metal interconnects.

In this process of forming the metal interconnects, the eddy current sensor is widely used in order to detect whether or not the unnecessary metal film has been removed (i.e., whether or not the unnecessary metal film remains). However, in the substrate having the multilayer interconnect structure, the interconnects existing under the metal film to be polished could affect the output signal of the eddy current sensor, thus preventing the detection of the remaining film.

In order to eliminate such an influence of the interconnects existing under the metal film, the following techniques have been used.

(1) An average of the output signal of the eddy current sensor obtained over the entire surface of the substrate is used to determine the film thickness.

(2) A minimum output signal obtained in a predefined zone on the substrate surface is used to determine the film thickness.

(3) A rotational speed ratio of the top ring to the polishing table is adjusted such that paths of the eddy current sensor described on the substrate surface are distributed substantially uniformly over the entire circumference of the substrate within a predetermined period of time (e.g., a moving average time of the output signal of the eddy current sensor).

However, it is difficult to obtain film thickness information for each of zones defined in the substrate surface with the use of the conventional techniques described above.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above drawbacks. It is a first object of the present invention to provide a polishing monitoring method and a polishing apparatus capable of calibrating an eddy current sensor without lowering an operating rate and capable of achieving highly-accurate film thickness monitoring.

It is a second object of the present invention to provide a method capable of obtaining film thickness information for each of zones defined in a substrate surface using an eddy current sensor while eliminating an influence of a metal lying under a film to be polished, and capable of determining a polishing end point of the substrate based on the film thickness information obtained.

It is a third object of the present invention to provide a polishing apparatus capable of performing such a polishing end point detection method.

A first aspect of the present invention for achieving the above object is to provide a method of monitoring a thickness of a conductive film on a substrate during polishing of the substrate with use of an eddy current sensor provided in a polishing table. The method includes: polishing the conductive film by pressing the substrate against a polishing surface on the rotating polishing table; obtaining output signal of the eddy current sensor during polishing; calculating an amount of output adjustment of the eddy current sensor using the output signal obtained when the substrate is not present above the eddy current sensor; with use of the amount of output adjustment, correcting the output signal obtained when the substrate is present above the eddy current sensor; and monitoring the thickness of the conductive film based on the corrected output signal.

According to the present invention, the process of polishing the conductive film on the substrate is started by pressing the substrate against the polishing surface on the rotating polishing table, and the output signal of the eddy current sensor is obtained during polishing of the substrate. Subsequently, the output signal obtained when the substrate is not present above the eddy current sensor is used to calculate the amount of the output adjustment of the eddy current sensor. Drift (translation) of the output signal of the eddy current sensor may occur due to a change in operating environment or a change in the eddy current sensor itself with time. Such drift can be removed from the output signal by correcting the output signal obtained when the substrate is present above the eddy current sensor with use of the amount of the output adjustment.

In a preferred aspect of the present invention, a resistance component and reactance component of impedance of electric circuit including a coil of the eddy current sensor are defined as coordinates on a coordinate system, the coordinates are rotated and moved such that a distance between an origin of the coordinate system and a point specified by the coordinates is reduced in accordance with a decrease in the thickness of the conductive film, and the output signal of the eddy current sensor is represented by the rotated and moved coordinates.

In a preferred aspect of the present invention, the correcting of the output signal is performed by moving the origin of the coordinate system.

According to the present invention, the drift can be removed from the output signal by translating the origin of the coordinate system on which the resistance component and the reactance component of the impedance including the coil of the eddy current sensor are plotted.

In a preferred aspect of the present invention, the monitoring the thickness of the conductive film based on the corrected output signal comprises monitoring the thickness of the conductive film based on the distance between the moved origin of the coordinate system and the point specified by the coordinates of the impedance.

In a preferred aspect of the present invention, the method further includes: obtaining resistance components and reactance components of impedance of electric circuit including a coil of the eddy current sensor at each of different thicknesses of the conductive film under conditions of various distances between a coil end of the eddy current sensor and the conductive film; plotting the resistance components and the reactance components onto rectangular coordinate axes; drawing preliminary measurement linear lines each connecting points specified by coordinates consisting of the resistance components and the reactance components at each thickness of the conductive film; and determining in advance a reference point at which the preliminary measurement linear lines intersect with each other.

In a preferred aspect of the present invention, the correcting of the output signal is performed by moving the reference point that has been determined in advance.

The output signal of the eddy current sensor may drift due to some causes, such as the change in the eddy current sensor itself with time. Such drift causes a change in the angle between the reference line extending through the reference point and the line connecting the output signal of the eddy current sensor to the reference point. According to the present invention, the preset reference point is moved by a distance corresponding to the amount of drift. Therefore, the drift can be removed from the output signal.

In a preferred aspect of the present invention, the monitoring the thickness of the conductive film based on the corrected output signal comprises monitoring the thickness of the conductive film based on an angle of a line connecting the moved reference point to a point specified by coordinates of the impedance.

In a preferred aspect of the present invention, the correcting of the output signal is performed by using an average of the output signal obtained when the substrate is not present above the eddy current sensor while the polishing table makes N revolutions.

In a preferred aspect of the present invention, the calculating of the amount of output adjustment comprises calculating the amount of output adjustment of the eddy current sensor using only output signal obtained from a region where a top ring for holding the substrate is not present on or above the polishing surface, among the output signal obtained when the substrate is not present above the eddy current sensor.

In a preferred aspect of the present invention, the calculating of the amount of output adjustment comprises calculating the amount of output adjustment of the eddy current sensor using only output signal obtained from a region where a dresser for dressing the polishing surface and an atomizer for cleaning the polishing surface are not present on or above the polishing surface, among the output signal obtained when the substrate is not present above the eddy current sensor.

According to the present invention, the output signal obtained when nothing exist on or above the polishing surface is used. Therefore, only the signal that is obtained in a region that does not affect the eddy current sensor can be used.

A second aspect of the present invention is to provide a method of monitoring a thickness of a conductive film on a substrate during polishing of the substrate with use of an eddy current sensor provided in a polishing table. The method includes: polishing the conductive film by pressing the substrate against a polishing surface on the rotating polishing table; obtaining resistance components and reactance components of impedance of electric circuit including a coil of the eddy current sensor at each of different thicknesses of the conductive film under conditions of various distances between a coil end of the eddy current sensor and the conductive film; plotting the resistance components and the reactance components onto rectangular coordinate axes; drawing preliminary measurement linear lines each connecting points specified by coordinates consisting of the resistance components and the reactance components at each thickness of the conductive film; determining a reference point at which the preliminary measurement linear lines intersect with each other; correcting the reference point using output signal of the eddy current sensor obtained during polishing, before polishing, or after polishing; and monitoring the thickness of the conductive film based on an angle of a line connecting the corrected reference point to a point specified by coordinates of the impedance.

The output signal of the eddy current sensor may drift due to some causes, such as the change in the eddy current sensor itself with time. Such drift of the output single of the eddy current sensor causes a change in the angle between the reference line extending through the reference point and the line connecting the output signal of the eddy current sensor to the reference point. According to the present invention, the predetermined reference point is corrected by the amount of drift, and the angle between the reference line extending through the corrected reference point and the line connecting the output signal of the eddy current sensor and the corrected reference point is calculated to thereby determine the thickness of the conductive film. In this manner, the angle before and after the drift can be kept at the same value by: determining the amount of drift of the output signal value of the eddy current sensor; and shifting the reference point by a distance corresponding to the amount of drift.

A third aspect of the present invention is to provide a polishing apparatus, including: a rotatable polishing table having a polishing surface; a top ring configured to press a substrate against the polishing surface to polish a conductive film on the substrate; an eddy current sensor arranged in the polishing table; and a monitoring device for monitoring a thickness of the conductive film based on output signal of the eddy current sensor, wherein the monitoring device is configured to obtain the output signal of the eddy current sensor during polishing, calculate an amount of output adjustment of the eddy current sensor using the output signal obtained when the substrate is not present above the eddy current sensor, correct the output signal obtained when the substrate is present above the eddy current sensor with use of the amount of output adjustment, and monitor the thickness of the conductive film based on the corrected output signal.

In a preferred aspect of the present invention, a resistance component and reactance component of impedance of electric circuit including a coil of the eddy current sensor are defined as coordinates on a coordinate system, the coordinates are rotated and moved such that a distance between an origin of the coordinate system and a point specified by the coordinates is reduced in accordance with a decrease in the thickness of the conductive film, and the output signal of the eddy current sensor is represented by the rotated and moved coordinates.

In a preferred aspect of the present invention, the monitoring device is configured to correct the output signal by moving the origin of the coordinate system.

In a preferred aspect of the present invention, the monitoring device is configured to monitor the thickness of the conductive film based on the distance between the moved origin of the coordinate system and the point specified by the coordinates of the impedance.

In a preferred aspect of the present invention, the monitoring device is configured to: obtain resistance components and reactance components of impedance of electric circuit including a coil of the eddy current sensor at each of different thicknesses of the conductive film under conditions of various distances between a coil end of the eddy current sensor and the conductive film; plot the resistance components and the reactance components onto rectangular coordinate axes; draw preliminary measurement linear lines each connecting points specified by coordinates consisting of the resistance components and the reactance components at each thickness of the conductive film; and determine in advance a reference point at which the preliminary measurement linear lines intersect with each other.

In a preferred aspect of the present invention, the monitoring device is configured to correct the output signal by moving the reference point that has been determined in advance.

In a preferred aspect of the present invention, the monitoring device is configured to monitor the thickness of the conductive film based on an angle of a line connecting the moved reference point to a point specified by coordinates of the impedance.

In a preferred aspect of the present invention, the monitoring device is configured to correct the output signal by using an average of the output signal obtained when the substrate is not present above the eddy current sensor while the polishing table makes N revolutions.

In a preferred aspect of the present invention, the monitoring device is configured to calculate the amount of output adjustment of the eddy current sensor using only output signal obtained from a region where a top ring for holding the substrate is not present on or above the polishing surface, among the output signal obtained when the substrate is not present above the eddy current sensor.

In a preferred aspect of the present invention, the monitoring device is configured to calculate the amount of output adjustment of the eddy current sensor using only output signal obtained from a region where a dresser for dressing the polishing surface and an atomizer for cleaning the polishing surface are not present on or above the polishing surface, among the output signal obtained when the substrate is not present above the eddy current sensor.

A fourth aspect of the present invention is to provide a method of detecting a polishing end point. The method includes: rotating a top ring and a polishing table while the top ring is pressing a substrate against a polishing pad on the polishing table to polish a film of the substrate; during polishing of the substrate, sweeping an eddy current sensor across a surface of the substrate; obtaining resistance component X and inductive reactance component Y of impedance of the eddy current sensor; plotting coordinates X and Y, which consist of the resistance component X and the inductive reactance component Y, onto a XY coordinate system on which a plurality of impedance areas are defined in advance, the plurality of impedance areas including a reference impedance area and at least one offset impedance area; calculating a plurality of film thickness index values in the plurality of impedance areas, respectively, using plural pairs of coordinates X and Y which belong respectively to the plurality of impedance areas; and determining polishing end points of the substrate in the plurality of impedance areas, respectively, using the plurality of film thickness index values.

A fifth aspect of the present invention is to provide a method of detecting a polishing end point. The method includes: rotating a top ring and a polishing table while the top ring is pressing a substrate against a polishing pad on the polishing table to polish a film on the substrate; obtaining output signal of an eddy current sensor provided in the polishing table while sweeping the eddy current sensor across a surface of the substrate; obtaining output signal of the eddy current sensor while sweeping the eddy current sensor across the surface of the substrate in the same path as that when the previous output signal is obtained; calculating a film thickness index value from the output signal of the eddy current sensor; and determining a polishing end point of the substrate from a change in the film thickness index value.

A sixth aspect of the present invention is to provide a method of detecting a polishing end point. The method includes: rotating a top ring and a polishing table while the top ring is pressing a substrate against a polishing pad on the polishing table to polish a film on the substrate; during polishing of the substrate, sweeping an eddy current sensor across a surface of the substrate, the eddy current sensor being provided in the polishing table; obtaining output signal of the eddy current sensor; producing a film thickness profile from the output signal of the eddy current sensor; judging whether a salient portion appearing in the film thickness profile is due to the film remaining on the substrate or due to metal lying under the film based on a change in position of the salient portion appearing in the film thickness profile; and determining a polishing end point of the substrate based on a size of the salient portion that appears due to the film remaining on the substrate.

A seventh aspect of the present invention is to provide a polishing apparatus including: a rotatable polishing table for supporting a polishing pad; a top ring configured to press a substrate against the polishing pad on the rotating polishing table while rotating the substrate; an eddy current sensor arranged in the polishing table so as to sweep a surface of the substrate; and a monitoring device for monitoring a film thickness of the substrate from output signal of the eddy current sensor, wherein the monitoring device is configured to obtain resistance component X and inductive reactance component Y of impedance of the eddy current sensor, plot coordinates X and Y, which consist of the resistance component X and the inductive reactance component Y, onto a XY coordinate system on which a plurality of impedance areas are defined in advance, the plurality of impedance areas including a reference impedance area and at least one offset impedance area, calculate a plurality of film thickness index values in the plurality of impedance areas, respectively, using plural pairs of coordinates X and Y which belong respectively to the plurality of impedance areas, and determine polishing end points of the substrate in the plurality of impedance areas, respectively, using the plurality of film thickness index values.

An eighth aspect of the present invention is to provide a polishing apparatus including: a rotatable polishing table for supporting a polishing pad; a top ring configured to press a substrate against the polishing pad on the rotating polishing table while rotating the substrate; an eddy current sensor arranged in the polishing table so as to sweep a surface of the substrate; and a monitoring device for monitoring a film thickness of the substrate from output signal of the eddy current sensor, wherein the monitoring device is configured to obtain output signal of the eddy current sensor while the eddy current sensor is sweeping the surface of the substrate, obtain output signal of the eddy current sensor while the eddy current sensor is sweeping the surface of the substrate in the same path as that when the previous output signal is obtained, calculate a film thickness index value from the output signal of the eddy current sensor, and determine a polishing end point of the substrate from a change in the film thickness index value.

According to the first to third aspects of the present invention, the calibration of the eddy current sensor can be performed on a software based on the output signal value of the eddy current sensor during polishing of the conductive film on the substrate, such as a semiconductor wafer. Therefore, accurate film thickness monitoring can be performed continuously without lowering the operating rate of the polishing apparatus.

According to the fourth and seventh aspects of the present invention, each time the output signal of the eddy current sensor is obtained, the coordinates consisting of the output signal X and Y are sorted into one of the plurality of impedance areas in accordance with the value of the coordinates. In other words, the sensor output signal is sorted based on the degree of the influence of the underlying metal into one of the plurality of impedance areas. By providing the plurality of impedance areas in advance, the variation in the sensor output signal (X, Y) can be divided, i.e., can be small. Therefore, the film thickness index value obtained from the sensor output signal in each impedance area decreases gradually with the polishing time. The plurality of impedance areas can be established in each zone defined in the wafer surface, so that film thickness information can be obtained in each zone in the wafer surface. Therefore, the polishing end point can be detected in each of the multiple zones defined in the wafer surface.

According to the fifth and eighth aspects of the present invention, the film thickness index value is obtained when the eddy current sensor sweeps the surface of the substrate in the same path. Therefore, regardless of the presence of the underlying metal, the film thickness index value decreases with the polishing time at each measuring point on the surface of the substrate. That is, the film thickness information can be obtained in each zone defined in the substrate surface. Therefore, the polishing end point can be detected in each of the multiple zones defined in the wafer surface.

According to the sixth aspect of the present invention, polishing of the substrate can be monitored based on the salient portion appearing due to the residual film. Therefore, the influence of the underlying metal is removed, and the accurate polishing end point can be detected

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a whole structure of a polishing apparatus;

FIG. 2 is a plan view showing a relationship between a polishing table, an eddy current sensor, and a wafer;

FIG. 3 is a diagram showing an equivalent circuit for illustrating a principle of the eddy current sensor;

FIG. 4 is a diagram showing a graph described by plotting X and Y, which vary with polishing time, onto a XY coordinate system;

FIG. 5 is a diagram showing a graph obtained by rotating the graph in FIG. 4 in a counterclockwise direction through 90 degrees and then translating the rotated graph;

FIG. 6 is a schematic view showing the eddy current sensor;

FIG. 7 is a view of a structural example of a sensor coil of the eddy current sensor shown in FIG. 6;

FIG. 8 is a schematic view of detailed structure of the eddy current sensor;

FIG. 9A is a view showing the whole structure of the polishing apparatus including a controller for the eddy current sensor;

FIG. 9B is an enlarged cross-sectional view of a part of the eddy current sensor;

FIG. 10A is a diagram showing a relationship between path of the eddy current sensor when sweeping (scanning) a wafer surface (i.e., a surface to be polished) and output of the eddy current sensor;

FIG. 10B is a diagram showing relationship between rotation of the polishing table and the output of the eddy current sensor;

FIG. 11 is a diagram showing drift (translation) of the output signal value of the eddy current sensor;

FIG. 12 is a diagraph showing an embodiment of a process flow of monitoring a change in thickness of a conductive film on the wafer while calibrating the output signal of the eddy current sensor;

FIG. 13 is a diagram for illustrating a step of calculating an amount of drift (an amount of correction);

FIG. 14 is a diagram for illustrating a step of moving an origin O of the XY coordinate system by a distance corresponding to the amount of drift (amount of correction);

FIG. 15 is a diagram showing another embodiment of the process flow of monitoring the change in the thickness of the conductive film on the wafer while calibrating the output signal of the eddy current sensor;

FIG. 16 is a diagram showing the drift (or translation) of the output signal value of the eddy current sensor;

FIG. 17 is a diagram for illustrating a step of shifting a reference point by a distance corresponding to the amount of correction;

FIG. 18 is a diagram showing paths of the eddy current sensor when scanning the wafer;

FIG. 19 is a diagram illustrating an influence of underlying interconnects that cause a change in film thickness index value obtained from the output signal of the eddy current sensor;

FIG. 20A is a diagram showing an impedance curve in a case where the influence of the underlying interconnects does not exist;

FIG. 20B is a diagram showing the film thickness index value obtained from the impedance curve shown in FIG. 20A;

FIG. 21A is a diagram showing an impedance curve in a case where the influence of the underlying interconnects exists;

FIG. 21B is a diagram showing the film thickness index value obtained from the impedance curve shown in FIG. 21A;

FIG. 22 is a diagram showing an example in which the wide impedance curve shown in FIG. 21B is divided into four impedance areas;

FIG. 23 is a diagram showing change in film thickness index values determined from plural pairs of coordinates X and Y wherein each pair belongs to each impedance area shown in FIG. 22;

FIG. 24 is a diagram showing a state in which first to third offset impedance areas overlap a reference impedance area;

FIG. 25 is diagram showing change in the film thickness index values determined from the coordinates X and Y belonging to the respective overlapping four impedance areas shown in FIG. 24;

FIG. 26 is a view of five zones defined in the surface of the wafer;

FIG. 27 is a diagram for illustrating a process of calculating an angle θ, which is the film thickness index value, from the output signal X and Y of the eddy current sensor;

FIG. 28 is a diagram showing an example in which the angle θ varies due to the existence of the underlying interconnect structure;

FIG. 29 is a diagraph illustrating an example of multiplying the angle calculated in relation to the offset impedance area by a coefficient;

FIG. 30 is a view showing paths of the eddy current sensor described on the wafer when a top ring rotates at a speed of 77 min⁻¹ and the polishing table rotates at a speed of 70 min⁻¹;

FIG. 31 is a diagram showing a change in film thickness profile on the same path of the eddy current sensor;

FIG. 32 is a diagram showing a residual film existing on a wafer and film thickness profiles of the wafer;

FIG. 33 is a diagram showing film thickness profiles of a wafer having the underlying interconnect structures and the residual film;

FIG. 34 is a schematic view of a table rotation detector and a top ring rotation detector;

FIG. 35 is a time chart illustrating a manner in which time-measuring devices measure times of rotation of the polishing table and the top ring upon receiving trigger signals;

FIG. 36 is a flowchart showing a process of detecting a polishing end point;

FIG. 37 is a diagram for illustrating a specific example of step 2 to step 5 shown in FIG. 36;

FIG. 38 is a cross-sectional view showing an example of the top ring shown in FIG. 1; and

FIG. 39 is a diagram showing a relationship between signal value of the eddy current sensor and polishing time (t) from when polishing of a semiconductor wafer (substrate) is started until a conductive film on the wafer is cleared (removed).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described below with reference to FIG. 1 to FIG. 38. The same or corresponding elements are denoted by the same reference numerals and repetitive explains will be omitted.

FIG. 1 is a schematic view showing a whole structure of a polishing apparatus according to the present invention. As shown in FIG. 1, the polishing apparatus has: a polishing table 1; and a top ring 10 for holding a wafer W (i.e., a substrate to be polished) and pressing the wafer W against a polishing pad 2 on the polishing table 1. The polishing table 1 is coupled to a polishing table rotating motor (not shown) through a table shaft 1 a, so that the polishing table 1 is rotated about the table shaft 1 a. The polishing table rotating motor is disposed below the polishing table 1. A polishing pad 2 is attached to an upper surface of the polishing table 1. The polishing pad 2 has an upper surface 2 a that provides a polishing surface for polishing the wafer W. A polishing liquid supply nozzle 3 is installed above the polishing table 1 so as to supply a polishing liquid (slurry) onto the polishing pad 2 on the polishing table 1. As shown in FIG. 1, an eddy current sensor 50 is embedded in the polishing table 1.

The top ring 10 is coupled to a top ring shaft 11, which is movable vertically relative to a top ring head 12. The vertical movement of the top ring shaft 11 causes the top ring 10 in its entirety to move vertically relative to the top ring head 12 and enables positioning of the top ring 10. The top ring shaft 11 is rotated by a top ring rotating motor (not shown). This rotation of the top ring shaft 11 causes the top ring 10 to rotate about the top ring shaft 11.

The top ring 10 is configured to be capable of holding the wafer W, such as a semiconductor wafer, on its lower surface. The top ring head 12 is configured to swing around a top ring head shaft 13, so that the top ring 10, holding the wafer W on the lower surface thereof, can move from a receiving position of the wafer W to a position above the polishing table 1. The top ring 10 holds the wafer W on its lower surface and presses the wafer W against the surface (i.e., the polishing surface) of the polishing pad 2. While pressing the wafer W, the top ring 10 and the polishing table 1 are rotated and the polishing liquid is supplied onto the polishing pad 2 from the polishing liquid supply nozzle 3 that is disposed above the polishing table 1. The polishing liquid contains abrasive grains, such as ceria (CeO₂) or silica (SiO₂). In this manner, the wafer W is pressed against the polishing pad 2 while the polishing liquid is supplied onto the polishing pad 2, and the wafer W and the polishing pad 2 are moved relative to each other to thereby polish a conductive film, e.g., a metal film, on the wafer. Examples of the metal film include Cu film, W film, Ta film, and Ti film.

As shown in FIG. 1, the polishing apparatus has a dressing unit 20 for dressing the polishing pad 2. This dressing unit 20 has a dresser arm 21, a dresser 22 rotatably mounted to a tip end of the dresser arm 21, a pivot shaft 23 coupled to the other end of the dressing arm 21, and a motor (not shown) as a driving device for causing the dresser arm 21 to pivot on (swing about) the pivot shaft 23. The dresser 22 has a lower portion that is constituted by a dressing member 22 a, which has a circular dressing surface. Hard particles, such as diamond particles or ceramic particles, are fixed to the dressing surface by means of electrodeposition or the like. A motor (not shown) for rotating the dresser 22 is provided in the dresser arm 21. The pivot shaft 23 is coupled to a vertical actuator (not shown), so that the dresser arm 21 is lowered by the vertical actuator to press the dressing member 22 a against the polishing surface 2 a of the polishing pad 2 to thereby dress the polishing pad 2. The dressing unit 20 is designed to be able to dress the polishing pad 2 when polishing of the wafer is not performed and further designed to be able to dress the polishing pad 2 even when polishing of the wafer is being performed.

FIG. 2 is a plan view showing a relationship between the polishing table 1, the eddy current sensor 50, and the wafer W. As shown in FIG. 2, the eddy current sensor 50 is arranged in such a position to pass through a center Cw of the wafer W when held by the top ring 10 during polishing of the wafer W. Symbol CT represents a center of rotation of the polishing table 1. For example, the eddy current sensor 50 is capable of detecting a thickness of the conductive film on the wafer W continuously on a sweep path (i.e., scan line) while sweeping under the wafer W.

Next, the eddy current sensor 50 provided in the polishing apparatus according to the present invention will be described in more detail. There are two types of eddy current sensors: a frequency type; and an impedance type. The frequency type is designed to detect a conductive film from a change in oscillatory frequency that is caused by an eddy current induced in the conductive film. The impedance type is designed to detect a conductive film from a change in impedance that is caused by an eddy current induced in the conductive film. Measurement information of the conductive film is obtained from the frequency or the impedance. The eddy current sensor 50 is installed in the polishing table 1 at a position near a surface of the polishing table 1 as shown in FIG. 1, and is arranged so as to face the wafer, to be polished, through the polishing pad in order to detect a change in the conductive film from the eddy current flowing through the conductive film on the wafer.

In the impedance-type eddy current sensor, signal output X and Y, phase, and resultant impedance Z are extracted as will be described below. Hereinafter, the impedance-type eddy current sensor will be described more specifically. The eddy current sensor 50 is configured to pass a high-frequency alternating current to a coil so as to induce the eddy current in the conductive film and detect the thickness of the conductive film from the change in the impedance due to a magnetic field produced by the induced eddy current.

FIG. 3 is a diagram showing a circuit for illustrating the principle of the eddy current sensor. When an AC power supply (a voltage E [V]) passes a high-frequency alternating current I₁ to a coil Q, magnetic lines of force, induced in the coil Q, pass through the conductive film. As a result, mutual inductance occurs between a sensor-side circuit and a conductive-film-side circuit, and an eddy current I₂ flows through the conductive film. This eddy current I₂ generates magnetic lines of force, which cause a change in an impedance of the sensor-side circuit. The eddy current sensor measures the thickness of the conductive film from the change in the impedance of the sensor-side circuit.

In the sensor-side circuit and the conductive-film-side circuit in FIG. 3, the following equations hold.

R ₁ I ₁ +L ₁ dI ₁ /dt+MdI ₂ /dt=E  (1)

R ₂ I ₂ +L ₂ dI ₂ /dt+MdI ₁ /dt=0  (2)

where M represents mutual inductance, R₁ represents equivalent resistance of the sensor-side circuit including the coil Q, L₁ represents self-inductance of the sensor-side circuit including the coil Q, R₂ represents equivalent resistance corresponding to eddy current loss, and L₂ represents self-inductance of the conductive film through which the eddy current flows.

Letting I_(n)=A_(n)e^(jωt) (sine wave), the above equations (1) and (2) are expressed as follows.

(R ₁ +jωL ₁)I ₁ +jωMI ₂ =E  (3)

(R ₂ +jωL ₂)I ₂ +jωMI ₁=0  (4)

From these equations (3) and (4), the following equations are derived.

$\begin{matrix} \begin{matrix} {I_{1} = {{E\left( {R_{2} + {{j\omega}\; L_{2}}} \right)}/\left\lbrack {{\left( {R_{1} + {{j\omega}\; L_{1}}} \right)\left( {R_{2} + {{j\omega}\; L_{2}}} \right)} + {\omega^{2}M^{2}}} \right\rbrack}} \\ {= {E/\left\lbrack {\left( {R_{1} + {{j\omega}\; L_{1}}} \right) + {\omega^{2}{M^{2}/\left( {R_{2} + {{j\omega}\; L_{2}}} \right)}}} \right\rbrack}} \end{matrix} & (5) \end{matrix}$

Thus, the impedance Φ of the sensor-side circuit is given by the following equation.

Φ=E/I ₁ =[R ₁+ω² M ² R ₂/(R ₂ ²+ω² L ₂ ²)]+jω[L ₁−ω² L ₂ M ²/(R ₂ ²+ω² L ₂ ²)]  (6)

Substituting X and Y for a real part (i.e., a resistance component) and an imaginary part (i.e., an inductive reactance component) respectively, the above equation (6) is expressed as follows.

Φ=X+jωY  (7)

FIG. 4 is a diagram showing a graph drawn by plotting X and Y, which change with a polishing time, on a XY coordinate system. The coordinate system shown in FIG. 4 is defined by a vertical axis as a Y-axis and a horizontal axis as a X-axis. Coordinates of a point T^(∞) are values of X and Y when a film thickness is infinity, i.e., R₂ is zero. Where electrical conductivity of a substrate can be neglected, coordinates of a point T0 are values of X and Y when the film thickness is zero, i.e., R₂ is infinity. A point Tn, specified by the values of X and Y, moves in a circular arc toward the point T0 as the film thickness decreases. A symbol k in FIG. 4 represents coupling coefficient, and the following relationship holds.

M=k(L ₁ L ₂)^(1/2)  (8)

FIG. 5 shows a graph obtained by rotating the graph in FIG. 4 through 90 degrees in a counterclockwise direction and further translating the resulting graph. Specifically, the point given by the coordinates X and Y is rotated about the origin O of the XY coordinate system, and the rotated point given by the coordinates are further moved so as to create a graph in which a distance between the origin O and the point specified by the coordinates X and Y decreases in accordance with the decrease in the film thickness. Hereinafter, a circular arc depicted by the point of the coordinates X and Y will be referred to as an impedance curve.

FIG. 5 shows the case where the graph in FIG. 4 is rotated through 90 degrees in the counterclockwise direction. Nevertheless, it is noted that the rotation angle is not limited to 90 degrees. For example, the rotation angle can be adjusted such that the Y-coordinate corresponding to an upper limit of the film thickness to be monitored is equal to the Y-coordinate of the point where the film thickness is zero. As shown in FIG. 5, the point Tn, which is specified by the values of X and Y, travels in a circular arc toward the point T0 as the film thickness decreases. During traveling, an impedance Z (=(X²+Y²)^(1/2)), which is expressed as a distance between the origin O of the XY coordinate system and the point Tn, decreases as the film thickness decreases, so long as the point Tn is not located near the point T∞. The output signal of the eddy current sensor 50 is sent to a monitoring device 55, which calculates the impedance Z as a monitoring signal that varies in accordance with the thickness of the conductive film. Therefore, once a relationship between the impedance Z and the film thickness is obtained in advance from experiences or experiments, the monitoring device 55 can grasp the change in the film thickness during polishing by monitoring the impedance Z.

Next, the eddy current sensor 50 will be described in more detail. FIG. 6 is a schematic view showing the eddy current sensor. This eddy current sensor 50 includes a sensor coil 102, an AC power supply 103 connected to the sensor coil 102, and a synchronous detector 105 configured to detect the resistance component X and the inductive reactance component Y of an electric circuit including the sensor coil 102 (the sensor-side circuit in FIG. 3). A conductive film mf, which is a film to be detected in its thickness, is a thin film on the wafer W. This thin film is made of a conductive material, such as copper, aluminum, gold, tungsten, tantalum, or titanium. A distance G between the sensor coil 102 and the conductive film mf is in a range of 0.5 mm to 5.0 mm, for example.

FIG. 7 is a view showing an example arrangement of the sensor coil of the eddy current sensor shown in FIG. 6. The sensor coil 102 includes a bobbin 111, and three coils 112, 113, and 114 wound on the bobbin 111. These coils 112, 113, and 114 form a three-layer coil. The center coil 112 is an exciting coil connected to the AC power supply 103. This exciting coil 112 produces a magnetic field by an alternating current supplied from the AC power supply 103 to thereby induce the eddy current in the conductive film on the wafer. The detection coil 113 is located above the exciting coil 112 (i.e., located at the conductive-film side). This detection coil 113 is configured to detect a magnetic flux generated by the eddy current flowing through the conductive film. The balance coil 114 is located at an opposite side of the detection coil 113.

The coils 113 and 114 preferably have the same number of turns (1 to 500 turns), while the number of tunes of the coil 112 is not limited particularly. The detection coil 113 and the balance coil 114 are connected in opposite phase to each other. When the conductive film is present near the detection coil 113, the magnetic flux produced by the eddy current flowing through the conductive film is interlinked with the detection coil 113 and the balance coil 114. Since the detection coil 113 is located closer to the conductive film than the coil 114, induced voltages produced in the coils 113 and 114 are unbalanced, whereby the interlinkage flux generated by the eddy current in the conductive film can be detected.

FIG. 8 is a schematic view showing the details of the eddy current sensor. The AC power supply 103 includes an oscillator, such as a quartz oscillator, generating a fixed frequency. For example, the AC power supply 103 supplies an alternating current having a fixed frequency of 1 to 50 MHz to the sensor coil 102. The alternating current generated by the AC power supply (AC signal source) 103 is supplied to the sensor coil 102 via a bandpass filter 120. A terminal of the sensor coil 102 outputs a signal, which is sent to the synchronous detector 105 via a bridge circuit 121 and a high-frequency amplifier 123. The synchronous detector 105 includes a cosine synchronous detection circuit 125 and a sine synchronous detection circuit 126. A phase shift circuit 124 generates two signals: in-phase component (0 degree) and quadrature component (90 degrees) from an oscillation signal generated by the AC power source 103. These two signals are sent to the cosine synchronous detection circuit 125 and the sine synchronous detection circuit 126, respectively. The synchronous detector 105 extracts the resistance component and the inductive reactance component of the impedance.

Low-pass filters 127 and 128 remove unwanted high-frequency components (e.g., 5 kHz or more) from the resistance component and the inductive reactance component outputted from the synchronous detector 105. As a result, the signal X as the resistance component and the signal Y as the inductive reactance component of the impedance are outputted from the eddy current sensor 50. The monitoring device 55 performs the same processes on the output signal X and Y as described with reference to FIG. 5 (e.g., the rotating process and the translating process) to thereby calculate the impedance Z (see FIG. 5) as a film thickness index value. Processing of the output signal X and Y of the eddy current sensor 50, e.g., the rotating process and the translating process, may be performed electrically in the eddy current sensor 50 or may be performed by the calculation in the monitoring device 55.

FIG. 9A and FIG. 9B are views showing an essential part of the polishing apparatus including the eddy current sensor 50. More specifically, FIG. 9A is a view showing a whole structure including a controller for the eddy current sensor 50, and FIG. 9B is an enlarged cross-sectional view of a part of the eddy current sensor. As shown in FIG. 9A, the polishing table 1 is rotatable about its own axis as indicated by arrow. The sensor coil 102 of the eddy current sensor 50 is embedded in the polishing table 1. This sensor coil 102 is a preamplifier-integrated sensor coil including the AC power source and the synchronous detection circuit. A connection cable of the sensor coil 102 extends through the table shaft 1 a of the polishing table 1 to the monitoring device 55 via a rotary joint 150 provided on an end of the table shaft 1 a. The monitoring device 55 is coupled to a control device (controller) 56.

As shown in FIG. 9B, the eddy current sensor 50 has a polishing-pad-side end surface that is covered with a coating material 152 of fluororesin, such as polytetrafluoroethylene. This coating material 152 can prevent the eddy current sensor from being removed together with the polishing pad 2 when the polishing pad 2 is removed from the polishing table 1. The polishing-pad-side end surface of the eddy current sensor 50 is located below an upper surface (a polishing-pad-side surface) of the polishing table 1 by a distance of 0 to 0.05 mm, so that the eddy current sensor 50 does not contact the polishing pad 2. The difference between the upper surface of the polishing table 1 and the upper end face of the eddy current sensor 50 should preferably be as small as possible. In the actual apparatus, the difference is typically set to around 0.02 mm. The height of the eddy current sensor 50 can be adjusted by any adjustment means, such as a shim (thin plate) 151 or a screw.

Next, a method of monitoring the thickness of the conductive film on the wafer during polishing in the polishing apparatus having the eddy current sensor will be described. FIG. 10A is a diagram showing a relationship between the path of the eddy current sensor 50 when sweeping (scanning) the surface, to be polished, of the wafer W and the output of the eddy current sensor 50. As shown in FIG. 10A, the eddy current sensor 50 is configured to output the signal value in response to the conductive film mf on the wafer W while passing under the wafer W as the polishing table 1 rotates.

FIG. 10B is a diagram showing a relationship between rotation of the polishing table 1 and the output of the eddy current sensor 50. In FIG. 10B, horizontal axis represents polishing time (t) and vertical axis represents the output value of the eddy current sensor 50. As shown in FIG. 10B, when the eddy current sensor 50 is in a region (A) in the wafer, the sensor output forms an approximately rectangular pulse as a result of the response to the conductive film mf on the wafer, and when the eddy current sensor 50 is in a region (B) that is outside the wafer, the sensor output is kept low in a constant level.

FIG. 11 is a diagram illustrating drift (translation) of the output signal value of the eddy current sensor. Drift (i.e., translation) of the output signal value of the eddy current sensor 50 could occur due to a change in operating environment, such as ambient temperature of the eddy current sensor or permeation of the liquid into the polishing pad, or a change in the eddy current sensor itself with time. As shown in FIG. 11, the output signal value of the eddy current sensor 50 may drift as indicated by a shift from a solid curve to a dotted curve. The drift of the output signal value of the eddy current sensor 50 causes a change in the impedance Z represented by a distance from the origin O of the XY coordinate system (hereinafter the impedance Z may be called distance Z). As a result, the relationship between the output signal value of the eddy current sensor and the film thickness varies.

Thus, in this embodiment, the monitoring device 55 calibrates the output signal of the eddy current sensor 50 so as to monitor an accurate change in the film thickness.

Next, a method of monitoring the change in the film thickness of the conductive film on the wafer while calibrating the output signal of the eddy current sensor 50 during polishing will be described.

FIG. 12 is a diagraph showing an embodiment of a process flow of monitoring the change in thickness of the conductive film on the wafer while calibrating the output signal of the eddy current sensor 50. As shown in FIG. 12, at step 1, the wafer W is held on the top ring 10, and the wafer W is pressed against the polishing pad 2 while the polishing table 1 and the top ring 10 are rotated, so that polishing of the conductive film on the wafer is started. As shown in FIG. 10A and FIG. 10B, during polishing, the eddy current sensor 50 passes through the region (A) in the wafer and the region (B) off the wafer as the polishing table 1 rotates. At step 2, the monitoring device 55 obtains data that are obtained by the eddy current sensor 50 when the sensor 50 is in the region (B) off the wafer. Specifically, after the polishing table 1 makes one or more revolutions since polishing is started, the monitoring device 55 obtains data of the region (B) off the wafer, and continues to obtain the data until the polishing table 1 makes N revolutions (N is integer). Then, at step 3, the monitoring device 55 calculates an amount of drift (i.e., an amount of correction) based on an average of the output values of the eddy current sensor 50 with respect to the region (B) off the wafer that are obtained until the polishing table 1 makes N revolutions.

The region off the wafer includes a region where the top ring is not present, a region where the dresser is not present, and a region where an atomizer and other structures are not present. That is, the region off the wafer is a region where nothing is present on the polishing table (the polishing pad).

FIG. 13 is a diagram for illustrating the step of calculating the amount of drift (the amount of correction). As indicated by curves in circular arc in FIG. 13, the output signal value of the eddy current sensor 50 may drift from a solid line to a dotted line.

This amount of drift (amount of correction) is calculated from the following equations.

ΔXa=X11−X1,ΔYa=Y11−Y1

where X11 and Y11 are average of the output values of the eddy current sensor 50 obtained in the off-wafer region (B), and X1 and Y1 are reference signal values for the correction, which are similar to values when the thickness of the conductive film is zero.

At step 4, the amount of drift (the amount of correction) calculated in the step 3 is registered (or stored). Then, at step 5, the origin O of the XY coordinate system is translated by a distance corresponding to the registered amount of drift (amount of correction).

FIG. 14 is a diagram for illustrating the step of moving the origin O of the XY coordinate system by a distance corresponding to the amount of drift (amount of correction). As shown in FIG. 14, the origin O of the XY coordinate system is translated from solid line to dotted line. More specifically, X axis and Y axis, indicated by solid lines, are translated by ΔXa and ΔYa to X axis and Y axis indicated by dotted lines. Then the distance Z from the origin O of the XY coordinate system defined by the X axis and the Y axis indicated by the dotted lines is calculated. The monitoring device 55 monitors the distance Z so as to grasp the change in the film thickness during polishing.

FIG. 15 is a diagram showing another embodiment of the process flow of monitoring the change in the thickness of the conductive film on the wafer while calibrating the output signal of the eddy current sensor 50. The drift of the output value of the eddy current sensor may affect a film thickness monitoring method other than the above-discussed method based on the distance Z as well. For example, Japanese laid-open patent publication No. 2005-121616 discloses in FIG. 13 a method of monitoring a change in film thickness during polishing from a change in an angle between a reference line extending through a reference point (central point) and a line extending through the reference point and a point specified by output signal (X component and Y component) of the eddy current sensor. This method includes the steps of: plotting, onto rectangular coordinate axes, resistance components (X components) and reactance components (Y components) of impedance obtained under conditions of various distances between a sensor coil end and the conductive film; drawing preliminary measurement linear lines each connecting points specified by coordinates consisting of the resistance components and the reactance components at each thickness of the conductive film; determining a reference point (central point) at which the preliminary measurement linear lines intersect with each other; and determining the thickness of the conductive film from an angle of a definitive measurement linear line connecting the point specified by the coordinates of the impedance to the reference point (central point). This method can monitor the change in the film thickness accurately regardless of a change in thickness of the polishing pad. However, even in this method, the variation in the output value of the eddy current sensor with time results in a change in the angle, thus causing a shift in relationship between the output signal value of the eddy current sensor and the film thickness.

FIG. 16 is a diagram showing the drift (translation) of the output signal value of the eddy current sensor. As indicated by curves in a circular arc in FIG. 16, the output signal value of the eddy current sensor 50 may drift from a solid line to a dotted line due to some causes including a change in the eddy current sensor itself with time. Such drift of the output signal value of the eddy current sensor causes the change in the angle between the reference line extending through the reference point (central point) and the line connecting the output signal of the eddy current sensor (the X component and the Y component) to the reference point (central point). Specifically, the angle is changed from “Angle 1” to “Angle 2”. The reference point is a point at which the preliminary measurement linear lines intersect with each other. The preliminary measurement linear lines are obtained by: plotting onto the rectangular coordinate axes the resistance components (X components) and the reactance components (Y components) of impedance obtained under the conditions of different distances between the sensor coil end and the conductive film; and drawing lines each connecting points specified by coordinates consisting of the resistance components and the reactance components at each thickness of the conductive film.

In this embodiment, as shown in FIG. 15, the processes from step 1 of starting the polishing process to step 4 of registering the amount of drift (amount of correction) are performed. The processes from the step 1 to the step 4 are the same as those in the process flow shown in FIG. 12. In the process flow shown in FIG. 15, at step 5, the reference point is shifted by a distance corresponding to the registered amount of drift (amount of correction).

FIG. 17 is a diagram for illustrating the step of shifting the reference point by a distance corresponding to the amount of correction. As shown in FIG. 17, the reference point is shifted by a distance corresponding to the amount of correction (ΔXa and ΔYa) as illustrated by arrow. Subsequently, at step 6, the angle (“Angle”) of the impedance curve is calculated using the corrected reference point. More specifically, the angle between the reference line extending through the corrected reference point (central point) and the line connecting the output signal (the X component and the Y component) of the eddy current sensor to the corrected reference point (central point) is calculated. The film thickness can be determined from the angle calculated. In this manner, the angle (“Angle”) before and after the drift can be kept at the same value by: determining the amount of drift of the output signal value of the eddy current sensor; and shifting the reference point by a distance corresponding to the amount of drift.

FIG. 18 is a diagram showing paths of the eddy current sensor 50 when scanning the wafer W. As the polishing table 1 rotates, the eddy current sensor 50 scans the surface of the wafer W while describing a path passing through the center C_(w) of the wafer W. Each time the eddy current sensor 50 sweeps the wafer surface, the eddy current sensor 50 measures the film thickness of the wafer W at plural measuring points. Typically, a rotational speed of the top ring 10 is different from a rotational speed of the polishing table 1. As a result, the path of the eddy current sensor 50 on the surface of the wafer W varies as the polishing table 1 rotates, as indicated by scan lines SL₁, SL₂, SL₃, . . . in FIG. 18. Even in this case, since the eddy current sensor 50 is located so as to sweep the center C_(w) of the wafer W as described above, the path of the eddy current sensor 50 passes through the center C_(w) of the wafer W in every revolution. In this embodiment, timing of film-thickness measuring by the eddy current sensor 50 is adjusted such that the film thickness at the center C_(w) of the wafer W is always monitored by the eddy current sensor 50 in every revolution. In FIG. 18, symbol MP_(m-n) represents an n-th measuring point on an m-th scan line SL_(m).

The output signal X and Y of the eddy current sensor 50 obtained at each measuring point is plotted as the coordinates X and Y on the XY coordinate system. The output signal X and Y of the eddy current sensor 50 varies in accordance with the film thickness. Specifically, as shown in FIG. 5, the distance (i.e., impedance) Z (=√(X²+Y²) between the origin O of the XY coordinate system and the point Tn defined by the coordinates X, Y decreases as the film thickness decreases. Therefore, it can be said that the distance Z determined from the output signal X and Y is an index value of the film thickness measured.

Although the film thickness of the wafer W can be determined from the output signal X and Y of the eddy current sensor 50, a great shift in the output signal of the eddy current sensor 50 may occur due to existence of metal under the film. The wafer having multilayer interconnect structure contains interconnects (i.e., metal) in each level of the multilayer interconnect structure. The interconnects lying under the film could affect the output signal of the eddy current sensor 50, thus preventing accurate measuring of the film thickness.

FIG. 19 is a diagram illustrating the influence of the underlying interconnects that cause the change in the film thickness index value Z obtained from the output signal of the eddy current sensor 50. A plurality of interconnect structures (e.g., integrated circuits) 200 lie under the film to be polished. In FIG. 19, these interconnect structures 200 are indicated by dotted lines because they are covered with the film to be polished. The film thickness index value Z decreases gradually in its entirety with the polishing time. However, the eddy current sensor 50 senses not only the film to be polished, but also the interconnect structures 200 lying under the film. As a result, the signal value of the eddy current sensor 50 is affected by the underlying interconnect structures 200.

As shown in FIG. 19, the path of the eddy current sensor 50 when the polishing table 1 is making N-th revolution differs from the path of the eddy current sensor 50 when the polishing table 1 is making N+1-th revolution. Accordingly, arrangement of the interconnect structures 200 sensed by the eddy current sensor 50 varies as the polishing table 1 rotates. This results in a variation in a film thickness profile (i.e., a film thickness distribution along radial direction of the wafer) which is produced from the film thickness index value Z. In this manner, the output signal X and Y of the eddy current sensor 50 is affected by the underlying interconnect structures 200, and the impedance curve described on the XY coordinate system fluctuates greatly.

FIG. 20A is a diagram showing the impedance curve in the case where the influence of the underlying interconnect structures does not exist, and FIG. 20B is a diagram showing the film thickness index value Z obtained from the impedance curve shown in FIG. 20A. When the influence of the underlying interconnect structures does not exist, a width of the fluctuation (indicated by dw) is small, although the impedance curve fluctuates to some degree due to system noise. In this case, as shown in FIG. 20B, the film thickness index value Z describes a line with a small width as well. In FIG. 20B, vertical axis represents the film thickness index value, and horizontal axis represents the polishing time. The film thickness index value Z decreases with the polishing time. Therefore, it is easy to detect a point where the film is removed, i.e., the polishing end point.

In contrast, FIG. 21A shows the impedance curve in the case where the influence of the underlying interconnect structures exists, and FIG. 21B shows the film thickness index value Z obtained from the impedance curve shown in FIG. 21A. When the influence of the underlying interconnect structures exists, the impedance curve fluctuates greatly. As a result, the width dw′ of the impedance curve becomes large. In this case, the film thickness index value Z describes a wide line as well. This makes it difficult to detect the polishing end point.

Thus, in this embodiment, the wide impedance curve as shown in FIG. 21A is divided along its longitudinal direction into a plurality of areas (which will hereinafter be referred to as impedance areas). The film thickness index valve is calculated in each of the impedance areas, and polishing of the wafer is monitored in each of the impedance areas based on the film thickness index value. FIG. 22 is a diagram showing an example in which the wide impedance curve shown in FIG. 21A is divided into four impedance areas. Hereinafter, the narrow impedance curve shown in FIG. 20A will be referred to as a reference impedance area, the wide impedance curve shown in FIG. 21A will be referred to as an initial impedance area, and the impedance area, other than the reference impedance area, of the divided impedance areas shown in FIG. 22 will be referred to as offset impedance area.

The four impedance areas, i.e., a reference impedance area r0, a first offset impedance area r1, a second offset impedance area r2, and a third offset impedance area r3, have the same width, which is the width dw of the reference impedance area r0 obtained under the condition that no influence of the underlying interconnect structures exists. However, the width of the offset impedance areas r1-r3 may differ slightly from the width of the reference impedance area r0.

The reference impedance area r0 is produced using only the output signal (X, Y) of the eddy current sensor 50 obtained in the center of the wafer. The eddy current sensor 50 always passes through the center of the wafer every time the polishing table 1 makes one revolution. Therefore, the film thickness index value Z obtained in the center of the wafer decreases with the polishing time, regardless of the presence of the underlying metal, such as interconnect structure. In other words, in the center of the wafer, the underlying metal does not affect the temporal change in the film thickness index value Z. Therefore, the narrow impedance area as shown in FIG. 20A can be produced from the sensor output signal (X, Y) obtained in the center of the wafer. This impedance area obtained in the center of the wafer is defined as the reference impedance area.

The width dw of the reference impedance area is given as a difference between the minimum distance and the maximum distance from a center of the circular arc of the reference impedance area. More specifically, the width dw of the reference impedance area is determined by: determining the center of the circular arc described by the reference impedance area using a known technique, such as least-squares method; and calculating the difference between the minimum distance and the maximum distance from the determined center. The width dw′ of the wide initial impedance area shown in FIG. 21A is also determined by calculation in the same manner. Then, the initial impedance area is divided based on the width dw of the reference impedance area. The number of impedance areas to be divided is determined from the width dw′ of the initial impedance area. Specifically, the number of impedance areas is determined by dividing the width dw′ of the initial impedance area by the width dw of the reference impedance area. In the example shown in FIG. 22, the four impedance areas r0, r1, r2, r3, including the reference impedance area, are produced. Depending on the width of the initial impedance area, any one of the impedance areas r1, r2, r3 may have a slightly different width.

The reference impedance area r0 and the offset impedance areas r1, r2, r3 are obtained in advance by polishing a wafer having the same structure as the wafer to be polished. Typically, one of a plurality of wafers with the same structure belonging to one lot is polished for producing the impedance areas r0, r1, r2, r3 in advance.

The plurality of impedance areas, that have been produced as described above, are defined on the XY coordinate system. Each time the output signal X and Y of the eddy current sensor is obtained, the coordinates consisting of the output signal X and Y are sorted into one of the four impedance areas in accordance with value of the output signal X and Y. In other words, based on the degree of influence of the underlying interconnect structure, the sensor output signal X and Y is sorted into one of the impedance areas r0, r1, r2, r3.

FIG. 23 is a diagram showing change in film thickness index values Z (each value Z is represented by the distance from the origin O to the point of the coordinates X and Y) determined from plural pairs of coordinates X and Y wherein each pair belongs to each impedance area. The film thickness index values Z describe four lines with the elapse of the polishing time. These four lines correspond to the four impedance areas r0, r1, r2, r3 (see FIG. 22) to which a position specified by the coordinates X and Y belongs. Polishing of the wafer is monitored based on each of the four film thickness index values corresponding to the four impedance areas, and the polishing end point is determined based on the change in each film thickness index value.

As shown in FIG. 24, the offset impedance areas r1, r2, r3 may be translated so as to overlap the reference impedance area r0. A distance of the translation for each offset impedance area is determined from a distance between the centers of the circular arcs of the four impedance areas, i.e., the width dw of the reference impedance area r0. Specifically, the first offset impedance area r1 is translated by a distance of dw×1, the second offset impedance area r2 is translated by a distance of dw×2, and the third offset impedance area r3 is translated by a distance of dw×3. With these operations, the first to third offset impedance areas r1, r2, r3 overlap the reference impedance area r0 as shown in FIG. 24.

FIG. 25 is diagram showing the change in the film thickness index values Z (the distance from the origin O to the point of the coordinates X and Y) determined from the plural pairs of coordinates X and Y wherein the plural pairs belong to the overlapping four impedance areas, respectively, shown in FIG. 24. As can be seen from FIG. 25, the four film thickness index values vary with the polishing time in the same manner, and the four film thickness index values at each polishing time are closer to each other, as compared with the film thickness index values shown in FIG. 23.

The polishing end point of the wafer is determined in each of the impedance areas. Specifically, the four film thickness index values corresponding to the four impedance areas are monitored separately. Points of time when the film thickness index values have reached respective predetermined threshold values are determined to be polishing end points in the impedance areas. The threshold values are set for the four film thickness index values, respectively. A final polishing end point may be a point of time when at least one of the four film thickness index values has reached the corresponding predetermined threshold value. For example, the polishing end point may be set to a point of time when the film thickness index value in the reference impedance area has reached the predetermined threshold value, or a point of time when the film thickness index values in all of the impedance areas have reached the threshold values. Further, the polishing end point may be set to a point of time when at least two of the film thickness index values have reached the predetermined threshold values.

The first to third offset impedance areas r1, r2, r3 are regions where the output signal of the eddy current sensor is under the influence of the underlying interconnect structure. However, in one offset impedance area, the output signal of the eddy current sensor is affected equally by the underlying interconnect structure. Therefore, the change in the film thickness index value in one offset impedance area reflects the progress of polishing of the wafer, regardless of the influence of the underlying interconnect structure. Accordingly, accuracy of detecting the polishing end point can be improved by monitoring polishing of the wafer in each of the divided impedance areas.

The above-discussed plurality of impedance areas can be produced for each of zones defined in the wafer surface. Therefore, the polishing end point in each zone of the wafer can be determined according to the above-described method. These zones can be defined as desired in the surface of the wafer. FIG. 26 shows one example. In this example, five zones C1, C2, C3, C4, and C5 are defined in the surface of the wafer W. The four film thickness index values are obtained in each one of the five zones C1 to C5. Therefore, 20 (5×4) film thickness index values are monitored during polishing of the wafer.

As discussed previously, by providing in advance the plurality of impedance areas including the reference impedance area, the variation in the sensor output signal (X, Y) can be divided, i.e., can be small. Therefore, the film thickness index value obtained from the sensor output signal in each impedance area decreases approximately with the polishing time. Because the plurality of impedance areas can be established in each zone defined in the wafer surface, film thickness information can be obtained in each zone in the wafer surface. Therefore, the polishing end point can be detected in each of the multiple zones defined in the wafer surface.

In the above-described embodiment, the film thickness index value Z (=√(X²+Y²) is calculated from the output signal X and Y of the eddy current sensor. In another embodiment, the film thickness index value may be an angle calculated from the output signal X and Y of the eddy current sensor. FIG. 27 is a diagram for illustrating a process of calculating the angle, which is the film thickness index value, from the output signal X and Y of the eddy current sensor. As shown in FIG. 27, an angle θ is defined as an angle between a reference line FL extending through a preset reference point (fixed point) F and a line connecting the reference point F to the point Tn determined from the output signal (the X component and the Y component) of the eddy current sensor. The angle θ varies as the point Tn moves, i.e., as the film thickness decreases. Therefore, this angle θ can be used as an index indicating the film thickness.

In general, the polishing pad 2 wears down gradually as the number of wafers polished increases. As shown in FIG. 1, the eddy current sensor 50 is embedded in the polishing table 1. Therefore, the distance between the wafer W and the eddy current sensor 50 varies with the wear of the polishing pad 2. It is known that the above-described angle θ varies depending on the film thickness without depending on the distance between the wafer W and the eddy current sensor 50 (see the Japanese laid-open patent publication No. 2005-121616).

However, as shown in FIG. 28, the angle θ may be changed due to the existence of the underlying interconnect structure and, as a result, may not reflect the film thickness accurately. Thus, the angles calculated for the offset impedance areas r1, r2, r3 are multiplied by preset coefficients which are such that the angles calculated for the offset impedance areas r1, r2, r3 become equal to the angle calculated for the reference impedance area r0, as shown in FIG. 29. These coefficients are established in advance for the offset impedance areas r1, r2, r3, respectively. Since the coefficients can vary depending on the structure of the wafer, the coefficients are determined from polishing results of wafer having the same structure as the wafer to be polished. Based on the corrected angles, the polishing end points can be detected in the impedance areas r0, r1, r2, r3, respectively.

Next, still another embodiment of the present invention will be described.

As shown in FIG. 19, the path of the eddy current sensor 50 when the polishing table 1 is making N-th revolution differs from that when the polishing table 1 is making N+1-th revolution. The arrangement of the interconnect structures 200 sensed by the eddy current sensor 50 varies as the polishing table 1 rotates. As a result, the film thickness profile obtained by the eddy current sensor 50 varies depending on the path of the eddy current sensor 50.

Typically, the rotational speed of the top ring 10 differs from the rotational speed of the polishing table 1. Under such a condition, the path, described on the wafer surface by the eddy current sensor 50, rotates about the center of the wafer. The path of the eddy current sensor 50 makes one revolution on the surface of the wafer while the polishing table 1 makes several revolutions. The number of revolutions of the polishing table 1 necessary for the sensor path to make one revolution on the surface of the wafer is determined by a rotational speed ratio of the top ring 10 to the polishing table 1.

FIG. 30 is a view showing the paths of the eddy current sensor described on the wafer W when the top ring 10 rotates at a speed of 77 min⁻¹ and the polishing table 1 rotates at a speed of 70 min⁻¹. As shown in FIG. 30, under this condition, the path of the eddy current sensor 50 rotates by 36 degrees each time the polishing table 1 makes one revolution. Therefore, the path of the eddy current sensor 50 makes one revolution each time the polishing table 1 makes ten revolutions. In this case, the sensor path when the polishing table 1 is making a first revolution is the same as the sensor path when the polishing table 1 is making an eleventh revolution.

FIG. 31 is a diagram showing a change in the film thickness profile on the same path of the eddy current sensor 50. The film thickness profile is a film thickness distribution along the radial direction of the wafer. When the eddy current sensor 50 measures the film thickness of the wafer along the same path, salient portions appear in the film thickness profile at the same positions due to the existence of the interconnect structures 200. Since the eddy current sensor 50 scans the same part of the wafer, the salient portions appear in the same positions. Therefore, regardless of the existence of the underlying interconnect structures 200, the film thickness profile in its entirety becomes smaller gradually with the polishing time. That is, the film thickness index value decreases with the polishing time in each film thickness measuring point on the wafer. Therefore, the polishing end point can be determined based on the change in the film thickness profile (i.e., the film thickness index value).

If the purpose of polishing is to remove the metal film from the wafer, the film thickness profile no longer varies after the metal film is removed from the wafer. This is because the eddy current sensor 50 does not react to the metal film any more. Therefore, a point of time when the film thickness profile stops changing (i.e., a point of time when the film thickness index value stops decreasing) can be determined to be the polishing end point. For example, the polishing end point may be a point of time when a difference between a current film thickness index value and a previous film thickness index value in the same location on the wafer is reduced to a predetermined value.

The change in the film thickness profile (i.e., the change in the film thickness index value) can be monitored in each of the plural zones defined in advance on the surface of the wafer as shown in FIG. 26. In each zone on the wafer, the film thickness index value decreases with the polishing time, regardless of the existence of the underlying interconnect structures. Therefore, the film thickness index value obtained from the eddy current sensor 50 can be used to detect the polishing end point in each zone on the wafer.

When the rotational speed of the top ring 10 differs from the rotational speed of the polishing table 1, there are a plurality of paths of the eddy current sensor 50 sweeping across the surface of the wafer. In the example shown in FIG. 30, there are ten paths since the sensor path makes one revolution about the center of the wafer W each time the polishing table 1 makes ten revolutions. The film thickness profile as shown in FIG. 31 may be produced for each of the ten paths. In this case also, it is possible to divide the wafer surface into five zones as in the example shown in FIG. 26. Therefore, in this case, 50 (10×5) polishing end points can be detected.

In the above-described example, the interconnect structure lying under the film affects the output signal of the eddy current sensor 50. The film to be polished can also cause the appearance of the salient portion in the film thickness profile if the film remains locally on the wafer. This example will be described with reference to FIG. 32. FIG. 32 is a diagram showing the film remaining locally on the wafer and the film thickness profiles of the wafer. Typically, the residual film is an annular film, as shown in FIG. 32. When such residual film exists on the wafer, the eddy current sensor 50 reacts to the residual film, and the film thickness index value becomes large. As a result, the salient portions appear in the film thickness profile as well as the example shown in FIG. 31.

The film thickness profile shown in FIG. 32 is different from the film thickness profile shown in FIG. 31 in that the salient portions always appear at constant positions regardless of the path of the eddy current sensor 50. This is because the residual film is an annular film extending in a circumferential direction of the wafer. These salient portions of the film thickness profile, indicating the existence of the residual film, appear at the same locations every time the polishing table 1 makes one revolution. The salient portions become smaller gradually with the polishing time, and disappear when the residual film is removed.

FIG. 33 is a diagram showing the film thickness profiles of the wafer having both the underlying interconnect structures and the residual film. The salient portions of the film thickness profile due to the underlying interconnect structures appear in different positions, so long as the sweep paths of the eddy current sensor 50 are not the same. In contrast, the salient portions of the film thickness profile due to the residual film appear in the same positions each time the polishing table 1 makes one revolution. Therefore, the monitoring device 55 can judge whether the salient portion is one due to the residual film or one due to the underlying interconnect structure from the position of the salient portion appearing in the film thickness profile. Further, the monitoring device 55 can determine the polishing end point from the change in size of the salient portion that is due to the residual film. For example, a point of time when the size of the salient portion is reduced to zero or a predetermined threshold value is determined to be the polishing end point.

The salient portion caused by the underlying interconnect structure and the salient portion caused by the residual film can be distinguished as follows. Each time the film thickness profile is obtained, the number of salient portions in the film thickness profile and the positions of the salient portions in the radial direction of the wafer are obtained. As can be seen from FIG. 33, every time the polishing table 1 makes one revolution, the salient portion due to the residual film appears in approximately the same position (or similar position) continuously, regardless of the sensor path. In contrast, the salient portion due to the underlying interconnect structure appears in approximately the same position (or similar position) with a constant cycle. Therefore, the salient portion appearing continuously in approximately the same position can be judged to be the salient portion due to the residual film, while the salient portion appearing in approximately the same position with a constant cycle can be judged to be the salient portion due to the underlying interconnect structure.

If the polishing table 1 and the top ring 10 rotate at the same rotational speed, the eddy current sensor 50 sweeps the wafer in the same path at all times. Therefore, the above-described method cannot be used to distinguish the salient portions. In such a case, it is possible to distinguish the salient portions based on whether or not a peak value of each salient portion decreases with the polishing time. More specifically, if the peak value of the salient portion no longer decreases after the polishing time has reached a certain point of time, such salient portion is judged to be the salient portion due to the underlying interconnect structure. In contrast, if the peak value of the salient portion decreases (even gradually) with the polishing time, such salient portion is judged to be the salient portion due to the residual film.

As discussed above, the number of sweep paths of the eddy current sensor 50 is determined by the ratio of the rotational speed of the top ring 10 to the rotational speed of the polishing table 1. In other words, by adjusting the rotational speed of the top ring 10 and the rotational speed of the polishing table 1, the eddy current sensor 50 can sweep the wafer in the same path each time the polishing table 1 makes desired revolutions. However, the top ring 10 and the polishing table 1 may not rotate at respective set speeds. In other words, there may be a difference between a set rotational speed and an actual rotational speed. Although this difference is small, the eddy current sensor 50 does not sweep the wafer in an expected path due to such difference. As a result, the eddy current sensor 50 cannot obtain the film-thickness profile with the salient portions appearing in the same position as shown in FIG. 31.

Thus, in one embodiment, actual times in which the top ring 10 and the polishing table 1 make one revolution are measured, so that the rotational speed of the top ring 10 and the rotational speed of the polishing table 1 are calculated from the measured actual times. FIG. 34 is a schematic view of a table rotation detector 210 for measuring a time of one revolution of the polishing table 1 and a top ring rotation detector 220 for measuring a time of one revolution of the top ring 10. The table rotation detector 210 has a sensor target 211 secured to a circumferential surface of the polishing table 1, a sensor 212 configured to sense the sensor target 211, and a time measuring device 213 coupled to the sensor 212.

The sensor target 211 is rotated together with the polishing table 1, while the sensor 212 is fixed in its position. The sensor 212 is disposed in proximity to the sensor target 211 so as to sense the sensor target 211 every time the polishing table 1 makes one revolution. When the sensor 212 senses the sensor target 211, the sensor 212 sends a trigger signal to the time measuring device 213, which measures a time from when the time measuring device 213 receives the trigger signal until the time measuring device 213 receives the next trigger signal. This measured time is a time required for the polishing table 1 to make one revolution.

The top ring rotation detector 220 has a sensor target 221 secured to the top ring 10, a sensor 222 configured to sense the sensor target 221, and a time measuring device 223 coupled to the sensor 222. The sensor 222 is secured to the top ring head 12 (see FIG. 1). Operations of the top ring rotation detector 220 are the same as those of the above-described table rotation detector 210 and will be omitted.

FIG. 35 is a time chart illustrating a manner in which the time measuring devices measure actual times per rotation upon receiving the trigger signals. Upon receiving the trigger signals, the time measuring devices 213 and 223 start measuring the time, and upon receiving the next trigger signals, the time measuring devices 213 and 223 stop measuring the time and simultaneously start measuring the time again. Since the trigger signal is inputted into the time measuring device 213 each time the polishing table 1 makes one revolution, a time interval between the trigger signal and the next trigger signal is the actual rotational time of the polishing table 1. Similarly, since the trigger signal is inputted into the time measuring device 223 each time the top ring 10 makes one revolution, a time interval between the trigger signal and the next trigger signal is the actual rotational time of the top ring 10.

The rotational speed (min⁻¹) of the polishing table 1 and the rotational speed (min⁻¹) of the top ring 10 can be calculated from the measured actual rotational times, respectively. In this manner, the actual rotational speed of the polishing table 1 and the actual rotational speed of the top ring 10 are obtained. Therefore, the rotational speed ratio of the top ring 10 to the polishing table 1 can be adjusted accurately. The eddy current sensor 50 can sweep the surface of the wafer in the same path accurately each time the polishing table 1 makes predetermined revolutions. Either the top ring rotation detector 220 or the table rotation detector 210 may be omitted. In this case, since the actual rotational speed of the top ring 10 or the polishing table 1 cannot be measured, the set rotational speed is used instead of the actual rotational speed.

Next, a process of detecting the polishing end point will be described with reference to FIG. 36. FIG. 36 is a flow chart showing the process of detecting the polishing end point. After polishing of the wafer is started, the eddy current sensor 50 scans the surface of the wafer each time the polishing table 1 makes one revolution, and outputs the signal X as the resistance component and the signal Y as the inductive reactance component of the impedance. The monitoring device 55 receives film thickness data including the output signal X and Y from the eddy current sensor 50 (step 1).

The monitoring device 55 receives the measured values of the rotational time of the polishing table 1 and the rotational time of the top ring 10 from the time measuring devices 213 and 223 (step 2). The monitoring device 55 calculates the actual rotational speed of the top ring 10 and the actual rotational speed of the polishing table 1 as discussed above. Further, the monitoring device 55 calculates the number of revolutions of the polishing table 1 required for the eddy current sensor 50 to describe the same path from the rotational speed ratio of the top ring 10 to the polishing table 1 (step 3).

The monitoring device 55 divides the film thickness data into a plurality of film thickness data groups in accordance with a plurality of zones (see FIG. 26) defined in advance on the wafer surface (step 4), sorts the film thickness data group with respect to each zone into a plurality of film thickness data in accordance with the path of the eddy current sensor 50 (step 5), and creates the film thickness profiles with respect to the sensor paths from the respective film thickness data.

A specific example of the above steps 2 to step 5 will be described with reference to FIG. 37. In this example, the actual time of the one revolution of the top ring 10 is 2000 milliseconds, and the actual time of the one revolution of the polishing table 1 is 1000 milliseconds. In this case, the rotational speed of the top ring 10 is determined to be 30 min⁻¹, the rotational speed of the polishing table 1 is determined to be 60 min⁻¹, and the eddy current sensor 50 sweeps the surface of the wafer twice each time the polishing table 1 makes one revolution. Therefore, the number of paths of the eddy current sensor 50 in this case is two. Five zones are defined in advance along each path on the surface of the wafer.

The film thickness data obtained when the polishing table 1 is making a 2N−1-th revolution are D1 _(2N-1), D2 _(2N-1), D3 _(2N-1), D4 _(2N-1), and D5 _(2N-1), and the film thickness data obtained when the polishing table 1 is making a 2N-th revolution are D1 _(2N), D2 _(2N), D3 _(2N), D4 _(2N), and D5 _(2N). N is a natural number. These film thickness data are divided into five data groups belonging to the respective five zones on the wafer: a first data group D1 _(2N-1), D1 _(2N); a second data group D2 _(2N-1), D2 _(2N); a third data group D3 _(2N-1), D3 _(2N); a fourth data group D4 _(2N-1), D4 _(2N); and a fifth data group D5 _(2N-1), D5 _(2N).

Further, each of the above data groups is sorted according to the sensor path. Specifically, the first data group is sorted into the film thickness data D1 _(2N-1) and the film thickness data D1 _(2N), the second data group is sorted into the film thickness data D2 _(2N-1) and the film thickness data D2 _(2N), the third data group is sorted into the film thickness data D3 _(2N-1) and the film thickness data D3 _(2N), the fourth data group is sorted into the film thickness data D4 _(2N-1) and the film thickness data D4 _(2N), and the fifth data group is sorted into the film thickness data D5 _(2N-1) and the film thickness data D5 _(2N). Then, the film thickness profiles are created from the respective film thickness data.

Referring back to FIG. 36, the monitoring device 55 compares a current film thickness obtained from each film thickness profile with a previous film thickness so as to obtain a change in the film thickness profile (step 6). More specifically, the monitoring device 55 judges whether or not a difference between the current film thickness and the previous film thickness is less than a preset value, or whether or not a decreasing rate of the film thickness is less than a preset value. In order to improve the accuracy of the polishing end point detection, these preset values are preferably determined based on the magnitude of system noise. When the difference between the current film thickness and the previous film thickness decreases below the preset value, or when the decreasing rate of the film thickness decreases below the preset value, the monitoring device 55 judges that the polishing process of the wafer has reached the end point (step 7).

In order to further improve the accuracy of the polishing end point detection, it is preferable that the monitoring device 55 judge that the polishing process of the wafer has reached the end point when the polishing end point in the step 7 is detected in the plural sensor paths. Alternatively, it is preferable that the monitoring device 55 judge that the polishing process of the wafer has reached the end point when the polishing end point in the step 7 is detected several times while the polishing table 1 makes several revolutions.

The above-described embodiments can be applied to the top ring capable of pressing the plurality of zones of the wafer separately against the polishing pad. FIG. 38 is an example of the top ring shown in FIG. 1. The top ring 10 has a top ring body 251 coupled to the top ring shaft 11 via a universal joint 250, and a retainer ring 252 provided on a lower portion of the top ring body 251.

The top ring 10 further has a flexible membrane 256 to be brought into contact with the wafer W, and a chucking plate 257 that holds the membrane 256. The membrane 256 and the chucking plate 257 are disposed below the top ring body 251. Four pressure chambers (air bags) P1, P2, P3, and P4 are provided between the membrane 256 and the chucking plate 257. The pressure chambers P1, P2, P3, and P4 are formed by the membrane 256 and the chucking plate 257. The central pressure chamber P1 has a circular shape, and the other pressure chambers P2, P3, and P4 have an annular shape. These pressure chambers P1, P2, P3, and P4 are in a concentric arrangement.

Pressurized fluid (e.g., pressurized air) is supplied into the pressure chambers P1, P2, P3, and P4 or vacuum is developed in the pressure chambers P1, P2, P3, and P4 by a pressure regulator 270 through fluid passages 261, 262, 263, and 264, respectively. The pressures in the pressure chambers P1, P2, P3, and P4 can be changed independently to thereby independently adjust loads on four zones of the wafer W: the central portion; an inner intermediate portion; an outer intermediate portion; and the peripheral portion. Further, by elevating or lowering the top ring 10 in its entirety, the retainer ring 252 can press the polishing pad 2 at a predetermined load.

A pressure chamber P5 is formed between the chucking plate 257 and the top ring body 251. Pressurized fluid is supplied into the pressure chamber P5 or vacuum is developed in the pressure chamber P5 by the pressure regulator 270 through a fluid passage 265. With this operation, the chucking plate 257 and the membrane 256 in their entirety can move up and down. The retainer ring 252 is arranged around the wafer W so as to prevent the wafer W from coming off the top ring 10 during polishing. The membrane 256 has an opening in a portion that forms the pressure chamber P3, so that the wafer W can be held by the top ring 10 via the vacuum suction by producing vacuum in the pressure chamber P3. Further, the wafer W can be released from the top ring 10 by supplying nitrogen gas or clean air into the pressure chamber P3.

The monitoring device 55 determines target values of internal pressure of the pressure chambers P1, P2, P3, and P4 based on the film thickness index values in the zones on the wafer surface corresponding to the pressure chambers P1, P2, P3, and P4. The monitoring device 55 sends command signals of the target values of internal pressure to the pressure regulator 270 so as to control the pressure regulator 270 such that the internal pressures of the pressure chambers P1, P2, P3, and P4 accord with the target values. In this manner, the top ring 10 having the multiple pressure chambers can press the respective zones on the wafer surface separately in accordance with the polishing progress, and can therefore polish the film uniformly.

The embodiments of the present invention have been descried above. Nevertheless, the present invention is not limited to the above-described embodiments and other embodiments can be made within a technical concept of the present invention. 

1. A method of monitoring a thickness of a conductive film on a substrate during polishing of the substrate with use of an eddy current sensor provided in a polishing table, said method comprising: polishing the conductive film by pressing the substrate against a polishing surface on the rotating polishing table; obtaining output signal of the eddy current sensor during polishing; calculating an amount of output adjustment of the eddy current sensor using the output signal obtained when the substrate is not present above the eddy current sensor; with use of the amount of output adjustment, correcting the output signal obtained when the substrate is present above the eddy current sensor; and monitoring the thickness of the conductive film based on the corrected output signal.
 2. The method according to claim 1, wherein: a resistance component and reactance component of impedance of electric circuit including a coil of the eddy current sensor are defined as coordinates on a coordinate system; the coordinates are rotated and moved such that a distance between an origin of the coordinate system and a point specified by the coordinates is reduced in accordance with a decrease in the thickness of the conductive film; and the output signal of the eddy current sensor is represented by the rotated and moved coordinates.
 3. The method according to claim 2, wherein said correcting of the output signal is performed by moving the origin of the coordinate system.
 4. The method according to claim 3, wherein said monitoring the thickness of the conductive film based on the corrected output signal comprises monitoring the thickness of the conductive film based on the distance between the moved origin of the coordinate system and the point specified by the coordinates of the impedance.
 5. The method according to claim 1; further comprising: obtaining resistance components and reactance components of impedance of electric circuit including a coil of the eddy current sensor at each of different thicknesses of the conductive film under conditions of various distances between a coil end of the eddy current sensor and the conductive film; plotting the resistance components and the reactance components onto rectangular coordinate axes; drawing preliminary measurement linear lines each connecting points specified by coordinates consisting of the resistance components and the reactance components at each thickness of the conductive film; and determining in advance a reference point at which the preliminary measurement linear lines intersect with each other.
 6. The method according to claim 5, wherein said correcting of the output signal is performed by moving the reference point that has been determined in advance.
 7. The method according to claim 6, wherein said monitoring the thickness of the conductive film based on the corrected output signal comprises monitoring the thickness of the conductive film based on an angle of a line connecting the moved reference point to a point specified by coordinates of the impedance.
 8. The method according to claim 1, wherein said correcting of the output signal is performed by using an average of the output signal obtained when the substrate is not present above the eddy current sensor while the polishing table makes N revolutions.
 9. The method according to claim 1, wherein said calculating of the amount of output adjustment comprises calculating the amount of output adjustment of the eddy current sensor using only output signal obtained from a region where a top ring for holding the substrate is not present on or above the polishing surface, among the output signal obtained when the substrate is not present above the eddy current sensor.
 10. The method according to claim 9, wherein said calculating of the amount of output adjustment comprises calculating the amount of output adjustment of the eddy current sensor using only output signal obtained from a region where a dresser for dressing the polishing surface and an atomizer for cleaning the polishing surface are not present on or above the polishing surface, among the output signal obtained when the substrate is not present above the eddy current sensor.
 11. A method of monitoring a thickness of a conductive film on a substrate during polishing of the substrate with use of an eddy current sensor provided in a polishing table, said method comprising: polishing the conductive film by pressing the substrate against a polishing surface on the rotating polishing table; obtaining resistance components and reactance components of impedance of electric circuit including a coil of the eddy current sensor at each of different thicknesses of the conductive film under conditions of various distances between a coil end of the eddy current sensor and the conductive film; plotting the resistance components and the reactance components onto rectangular coordinate axes; drawing preliminary measurement linear lines each connecting points specified by coordinates consisting of the resistance components and the reactance components at each thickness of the conductive film; determining a reference point at which the preliminary measurement linear lines intersect with each other; correcting the reference point using output signal of the eddy current sensor obtained during polishing, before polishing, or after polishing; and monitoring the thickness of the conductive film based on an angle of a line connecting the corrected reference point to a point specified by coordinates of the impedance.
 12. A polishing apparatus, comprising: a rotatable polishing table having a polishing surface; a top ring configured to press a substrate against the polishing surface to polish a conductive film on the substrate; an eddy current sensor arranged in said polishing table; and a monitoring device for monitoring a thickness of the conductive film based on output signal of said eddy current sensor, wherein said monitoring device is configured to obtain the output signal of said eddy current sensor during polishing, calculate an amount of output adjustment of said eddy current sensor using the output signal obtained when the substrate is not present above said eddy current sensor, with use of the amount of output adjustment, correct the output signal obtained when the substrate is present above said eddy current sensor, and monitor the thickness of the conductive film based on the corrected output signal.
 13. The polishing apparatus according to claim 12, wherein: a resistance component and reactance component of impedance of electric circuit including a coil of said eddy current sensor are defined as coordinates on a coordinate system; the coordinates are rotated and moved such that a distance between an origin of the coordinate system and a point specified by the coordinates is reduced in accordance with a decrease in the thickness of the conductive film; and the output signal of said eddy current sensor is represented by the rotated and moved coordinates.
 14. The polishing apparatus according to claim 13, wherein said monitoring device is configured to correct the output signal by moving the origin of the coordinate system.
 15. The polishing apparatus according to claim 14, wherein said monitoring device is configured to monitor the thickness of the conductive film based on the distance between the moved origin of the coordinate system and the point specified by the coordinates of the impedance.
 16. The polishing apparatus according to claim 12, wherein said monitoring device is configured to: obtain resistance components and reactance components of impedance of electric circuit including a coil of the eddy current sensor at each of different thicknesses of the conductive film under conditions of various distances between a coil end of the eddy current sensor and the conductive film; plot the resistance components and the reactance components onto rectangular coordinate axes; draw preliminary measurement linear lines each connecting points specified by coordinates consisting of the resistance components and the reactance components at each thickness of the conductive film; and determine in advance a reference point at which the preliminary measurement linear lines intersect with each other.
 17. The polishing apparatus according to claim 16, wherein said monitoring device is configured to correct the output signal by moving the reference point that has been determined in advance.
 18. The polishing apparatus according to claim 17, wherein said monitoring device is configured to monitor the thickness of the conductive film based on an angle of a line connecting the moved reference point to a point specified by coordinates of the impedance.
 19. The polishing apparatus according to claim 12, wherein said monitoring device is configured to correct the output signal by using an average of the output signal obtained when the substrate is not present above the eddy current sensor while the polishing table makes N revolutions.
 20. The polishing apparatus according to claim 12, wherein said monitoring device is configured to calculate the amount of output adjustment of the eddy current sensor using only output signal obtained from a region where a top ring for holding the substrate is not present on or above the polishing surface, among the output signal obtained when the substrate is not present above the eddy current sensor.
 21. The polishing apparatus according to claim 20, wherein said monitoring device is configured to calculate the amount of output adjustment of the eddy current sensor using only output signal obtained from a region where a dresser for dressing the polishing surface and an atomizer for cleaning the polishing surface are not present on or above the polishing surface, among the output signal obtained when the substrate is not present above the eddy current sensor.
 22. A method of detecting a polishing end point, said method comprising: rotating a top ring and a polishing table while the top ring is pressing a substrate against a polishing pad on the polishing table to polish a film of the substrate; during polishing of the substrate, sweeping an eddy current sensor across a surface of the substrate; obtaining resistance component X and inductive reactance component Y of impedance of the eddy current sensor; plotting coordinates X and Y, which consist of the resistance component X and the inductive reactance component Y, onto a XY coordinate system on which a plurality of impedance areas are defined in advance, the plurality of impedance areas including a reference impedance area and at least one offset impedance area; calculating a plurality of film thickness index values in the plurality of impedance areas, respectively, using plural pairs of coordinates X and Y which belong respectively to the plurality of impedance areas; and determining polishing end points of the substrate in the plurality of impedance areas, respectively, using the plurality of film thickness index values.
 23. The method according to claim 22, wherein said plurality of impedance areas are obtained by: polishing a substrate identical to the substrate to be polished; obtaining the resistance component X and the inductive reactance component Y during polishing of the identical substrate; plotting coordinates X and Y, which consist of the resistance component X and the inductive reactance component Y obtained, onto the XY coordinate system to form an initial impedance area; and dividing the initial impedance area along its longitudinal direction.
 24. The method according to claim 22, wherein said reference impedance area is obtained by: polishing a substrate identical to the substrate to be polished; obtaining the resistance component X and the inductive reactance component Y in a center of the identical substrate; and plotting coordinates X and Y, which consist of the resistance component X and the inductive reactance component Y obtained, onto the XY coordinate system.
 25. The method according to claim 22, wherein said offset impedance area has the same width as that of the reference impedance area.
 26. The method according to claim 22, wherein the film thickness index value is represented by a distance between a point specified by the coordinates X and Y and an origin of the XY coordinate system.
 27. The method according to claim 22, further comprising: translating the offset impedance area until the offset impedance area overlaps the reference impedance area.
 28. The method according to claim 22, wherein the film thickness index value is represented by an angle between a linear line connecting a point specified by the coordinates X and Y to a predetermined reference point and a predetermined reference line extending through the reference point.
 29. The method according to claim 28, further comprising: multiplying the angle obtained in the offset impedance area by a coefficient such that the angle obtained in the offset impedance area is equal to the angle obtained in the reference impedance area.
 30. A method of detecting a polishing end point, said method comprising: rotating a top ring and a polishing table while the top ring is pressing a substrate against a polishing pad on the polishing table to polish a film on the substrate; obtaining output signal of an eddy current sensor provided in the polishing table while sweeping the eddy current sensor across a surface of the substrate; obtaining output signal of the eddy current sensor while sweeping the eddy current sensor across the surface of the substrate in the same path as that when the previous output signal is obtained; calculating a film thickness index value from the output signal of the eddy current sensor; and determining a polishing end point of the substrate from a change in the film thickness index value.
 31. The method according to claim 30, wherein the number of revolutions of the polishing table required for the eddy current sensor to describe the same path is calculated from a ratio of a rotational speed of the top ring to a rotational speed of the polishing table.
 32. The method according to claim 31, wherein a time required for the polishing table to make one revolution is measured, and the rotational speed of the polishing table is calculated from the measured time.
 33. The method according to claim 31, wherein a time required for the top ring to make one revolution is measured, and the rotational speed of the top ring is calculated from the measured time.
 34. The method according to claim 30, further comprising: sorting the output signal of the eddy current sensor in accordance with a plurality of zones defined in advance on the surface of the substrate, wherein said calculating of the film thickness index value comprises calculating film thickness index values for the respective zones on the substrate from the sorted output signal.
 35. A method of detecting a polishing end point, said method comprising: rotating a top ring and a polishing table while the top ring is pressing a substrate against a polishing pad on the polishing table to polish a film on the substrate; during polishing of the substrate, sweeping an eddy current sensor across a surface of the substrate, said eddy current sensor being provided in the polishing table; obtaining output signal of the eddy current sensor; producing a film thickness profile from the output signal of the eddy current sensor; judging whether a salient portion appearing in the film thickness profile is due to the film remaining on the substrate or due to metal lying under the film based on a change in position of the salient portion appearing in the film thickness profile; and determining a polishing end point of the substrate based on a size of the salient portion that appears due to the film remaining on the substrate.
 36. A polishing apparatus, comprising: a rotatable polishing table for supporting a polishing pad; a top ring configured to press a substrate against the polishing pad on said rotating polishing table while rotating the substrate; an eddy current sensor arranged in said polishing table so as to sweep a surface of the substrate; and a monitoring device for monitoring a film thickness of the substrate from output signal of said eddy current sensor, wherein said monitoring device is configured to obtain resistance component X and inductive reactance component Y of impedance of said eddy current sensor, plot coordinates X and Y, which consist of the resistance component X and the inductive reactance component Y, onto a XY coordinate system on which a plurality of impedance areas are defined in advance, the plurality of impedance areas including a reference impedance area and at least one offset impedance area, calculate a plurality of film thickness index values in the plurality of impedance areas, respectively, using plural pairs of coordinates X and Y which belong respectively to the plurality of impedance areas, and determine polishing end points of the substrate in the plurality of impedance areas, respectively, using the plurality of film thickness index values.
 37. A polishing apparatus, comprising: a rotatable polishing table for supporting a polishing pad; a top ring configured to press a substrate against the polishing pad on said rotating polishing table while rotating the substrate; an eddy current sensor arranged in said polishing table so as to sweep a surface of the substrate; and a monitoring device for monitoring a film thickness of the substrate from output signal of said eddy current sensor, wherein said monitoring device is configured to obtain output signal of said eddy current sensor while said eddy current sensor is sweeping the surface of the substrate, obtain output signal of said eddy current sensor while said eddy current sensor is sweeping the surface of the substrate in the same path as that when the previous output signal is obtained, calculate a film thickness index value from the output signal of said eddy current sensor, and determine a polishing end point of the substrate from a change in the film thickness index value. 